Combination thermal wave and optical spectroscopy measurement systems

ABSTRACT

A combination metrology tool is disclosed which is capable of obtaining both thermal wave and optical spectroscopy measurements on a semiconductor wafer. In a preferred embodiment, the principal combination includes a thermal wave measurement and a spectroscopic ellipsometric measurement. These measurements are used to characterize ion implantation processes in semiconductors over a large dosage range.

TECHNICAL FIELD

[0001] The subject invention relates to a method and apparatusparticularly suited for the analysis ion implantation at higher doses onsemiconductor samples.

BACKGROUND OF THE INVENTION

[0002] There is a great need in the semiconductor industry for metrologyequipment which can provide high resolution, nondestructive evaluationof product wafers as they pass through various fabrication stages. Inrecent years, a number of products have been developed for thenondestructive evaluation of semiconductor samples. One such product hasbeen successfully marketed by the assignee herein under the trademarkTherma-Probe. This device incorporates technology described in thefollowing U.S. Pat. Nos. 4,634,290; 4,636,088, 4,854,710 and 5,074,669.The latter patents are incorporated herein by reference.

[0003] The Therma-Probe device monitors ion implant dose using thermalwave technology. In this device, an intensity modulated pump laser beamis focused on the sample surface for periodically exciting the sample.In the case of a semiconductor, thermal and plasma waves are generatedin the sample which spread out from the pump beam spot. These wavesreflect and scatter off various features and interact with variousregions within the sample in a way which alters the flow of heat and/orplasma from the pump beam spot.

[0004] The presence of the thermal and plasma waves has a direct effecton the reflectivity at the surface of the sample. Features and regionsbelow the sample surface which alter the passage of the thermal andplasma waves will therefore alter the optical reflective patterns at thesurface of the sample. By monitoring the changes in reflectivity of thesample at the surface, information about characteristics below thesurface can be investigated.

[0005] In the basic device, a second laser is provided for generating aprobe beam of radiation. This probe beam is focused colinearly with thepump beam and reflects off the sample. A photodetector is provided formonitoring the power of reflected probe beam. The photodetectorgenerates an output signal which is proportional to the reflected powerof the probe beam and is therefore indicative of the varying opticalreflectivity of the sample surface.

[0006] The output signal from the photodetector is filtered to isolatethe changes which are synchronous with the pump beam modulationfrequency. In the preferred embodiment, a lock-in detector is used tomonitor the magnitude and phase of the periodic reflectivity signal.This output signal is conventionally referred to as the modulatedoptical reflectivity (MOR) of the sample.

[0007] Thermal wave technology is well suited for measuring latticedamage in crystalline materials and, therefore, serves as an excellenttechnology for monitoring the ion implant process in semiconductormaterials. It is also known that optical methods, such as spectroscopicreflectance and spectroscopic ellipsometry, are sensitive to latticedamage through the effects of such damage on the optical properties ofthe material being implanted.

[0008] Typically, thermal waves are more sensitive in the region of lowimplantation, i.e. less than 10¹² ions/cm² (arsenic at 30 KEV) than theoptical methods. In the range of 10¹² through 10¹⁴ ions/cm², it appearsthat optical and thermal waves are comparable in their ability to detectchanges in lattice damage. At higher doses (of the same implant),amorphization sets in and the thermal wave signal is no longer monotonicwith increasing dose and cannot be used reliably to monitor the implantprocess. In this high dose region, the optical methods are verysensitive and can unambiguously measure the thickness of the amorphouslayer.

[0009] There is, however, damage above and below the amorphous layerwhich can still make an accurate measurement of total damage in theimplanted material difficult using only optical methods. Morespecifically, the implantation process at high doses will create a largedamaged region with a relatively smaller layer of amorphous material inthe center thereof. This occurs because during the implantation process,the ions travel very quickly as they first strike the lattice. The fastpassage through the lattice can result in little or no damageimmediately beneath the surface. As the ions begin to slow down, thedamage increases until at a certain depth, the damage is sufficient toproduce amorphization. Amorphization represents the peak damage to thelattice. Ions which travel beyond the amorphous layer will cause furtherdamage, but below the threshold for amorphization. Semiconductormanufacturers are interested in knowing both the thickness of theamorphous region, as well as the total extent of damage to the latticewhich would include the damaged regions both above and below theamorphous layer.

[0010] Thermal waves are intrinsically more sensitive to total damagethan the optical methods. Therefore, by combining thermal waves withspectroscopic measurements, one can provide a means for sensitive andunambiguous monitoring of the ion implant process throughout the entirerange. More specifically, one can use the data derived from the thermalwave measurements to provide an indication of the full extent of thedamaged region. Data obtained from a spectroscopic measurements can beused to provide an indication of the thickness of the amorphous layer.By combining these two sets of measurements, one can provide an accurateprofile of the damage as a function of depth below the surface of thesemiconductor wafer.

[0011] The concept of combining thermal wave measurements with otheroptical measurements is disclosed in prior U.S. Pat. No. 5,978,074,issued Nov. 2, 1999, and is assigned to the same assignee as the subjectinvention and is incorporated herein by reference. This patent describesthe need to obtain additional measurements where the sample is morecomplex. In one aspect of that patent disclosure, a conventional thermalwave detection system was modified to increase the amount of data whichcould be obtained. For example, a steering system was provided forvarying the distance between the pump and probe beam spots asmeasurements were taken. Another approach was to obtain a sequence ofmeasurements at various pump beam modulation frequencies.

[0012] The prior patent also discussed the advantages of combiningspectroscopic reflectivity measurements with the thermal wavemeasurements. Various additional measurements were suggested includingthe assignee's proprietary beam profile reflectometry and beam profileellipsometry techniques. The latter two approaches are described in U.S.Pat. Nos. 4,999,014 and 5,181,080, both of which are incorporated hereinby reference.

[0013] The principal application for the tool described in U.S. Pat. No.5,87,974 relates to measuring thin metal films formed on semiconductorsamples. The latter patent did not disclose the advantages of combiningthermal wave measurements with spectroscopic ellipsometry measurements.Further, the latter patent did not discuss the specific concept of usinga thermal wave measurement to provide information on the full extent ofa damage layer, while using another optical measurement to provide anindication of the amorphous layer.

[0014] Accordingly, it is an object of the subject invention to providea new method and apparatus which provides additional measurementcapabilities.

[0015] It is another object of the subject invention to provide a methodand apparatus particularly suited to evaluating high dopantconcentrations in semiconductors.

[0016] It is a further object of the subject invention to provide amethod and apparatus which combines measurements of modulated opticalreflectivity with modulated spectroscopic ellipsometry.

SUMMARY OF THE INVENTION

[0017] In accordance with these and other objects, the subject inventionincludes a method wherein a sample is characterized through acombination of measurements which include both a thermal or plasma wavemeasurement and a spectroscopic measurement. The thermal/plasma wavemeasurement is obtained by periodically exciting a region on the samplewith an intensity modulated pump beam. A probe beam is directed to aregion on the sample surface which has been periodically excited.Changes in power of the reflected probe beam are monitored to obtain themodulated optical reflectometry of the sample.

[0018] In accordance with the subject method, a separate spectroscopicmeasurement is also obtained. To obtain this measurement, apolychromatic light source generates a polychromatic probe beam which isdirected to reflect off the sample. The intensity of the reflectedpolychromatic probe beam can be measured to obtain spectroscopicreflectance data. Alternatively, or in addition, the change inpolarization state of the polychromatic probe beam can be measured toobtain ellipsometric information. Additional measurement technologiescan also be employed.

[0019] In accordance with the subject invention, data corresponding tothe modulated optical reflectivity signal is combined with thespectroscopic data to more accurately characterize the sample. In onepreferred embodiment, the system is used to more fully characterize highdosage levels of ion implantation in a semiconductor wafer. In thisapproach, the modulated optical reflectivity signal is useful forcharacterizing the full extent of the damaged region, while thespectroscopic ellipsometric information is used to characterize theextent of the amorphous layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a schematic diagram of the apparatus for carrying outthe methods of the subject invention.

[0021]FIG. 2 is a graph illustrating the damage layer thickness across afull ion implant dose range as measured by a spectroscopic ellipsometer.

[0022]FIG. 3 is a graph illustrating the thermal wave response across afull ion implant dose range.

[0023]FIG. 4 is a statistical process control plot of long termmonitoring of a silicon wafers implanted with arsenic using the threedifferent measurement technologies.

[0024]FIG. 5 is a statistical process control plot of long termmonitoring of a silicon wafers implanted with arsenic using the threedifferent measurement technologies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025]FIG. 1 is a simplified diagram of the basic components of anapparatus 10 which can be used to take the measurements useful inapplying the methods of the subject invention. The apparatus isparticularly suited for measuring characteristics of semiconductorwafers 20. In one important aspect of the invention, the device is usedto characterize levels of ion implantation in the wafer. The devicecould also be used to characterize properties of one or more thin filmlayers 22 on top of the wafer.

[0026] In accordance with the subject invention, the apparatus includesa first measurement system for generating thermal and/or plasma waversand monitoring the propagation of these waves in the sample. Thisportion of the system includes a pump laser 30 for exciting the sampleand a probe laser 32 for monitoring the sample. Gas, solid state orsemiconductor lasers can be used. As described in the assignee's earlierpatents, other means for exciting the sample can include differentsources of electromagnetic radiation or particle beams such as from anelectron gun.

[0027] In the preferred embodiment, semiconductor lasers are selectedfor both the pump and probe lasers due to their reliability and longlife. In the illustrated embodiment, pump laser 30 generates a nearinfrared output beam 34 at 790 nm while probe laser 32 generates avisible output beam 36 at 670 nm. Suitable semiconductor lasers for thisapplication include the Mitsubishi ML6414R (operated at 30 mW output)for the pump laser and a Toshiba Model 9211 (5 mW output) for the probelaser. The outputs of the two lasers are linearly polarized. The beamsare combined with a dichroic mirror 38. It is also possible to use twolasers with similar wavelengths and rely on polarization discriminationfor beam combining and splitting.

[0028] Pump laser 30 is connected to a power supply 40 which is underthe control of a processor 42. The output beam of laser 30 is intensitymodulated through the output of power supply 40. The modulationfrequency has a range running from 100 KHz to 100 MHz. In the preferredembodiment, the modulation frequency can be set up to 125 MHz. Asdescribed in the above cited patents, if an ion laser is used togenerate the pump beam, the intensity modulation can be achieved by aseparate acousto-optic modulator.

[0029] Prior to reaching the beam combining mirror 36, the probe beam 34passes through a tracker 46. Tracker 46 is used to control the lateralposition of beam 34 with respect to the probe beam. In somemeasurements, the two beams will be positioned so that the spots willoverlap on the sample surface. In addition, measurements can be taken atvarious spacings between the pump and probe beam spots. Measurements atdifferent spatial separations are discussed in greater detail in U.S.Pat. No. 5,978,074.

[0030] The beams are directed down to the sample 20 through a microscopeobjective 50. Objective 50 has a high n.a., on the order of 0.9, and iscapable of focusing the beam to a spot size on the order of a fewmicrons and preferably close to one micron in diameter. The spacingbetween the objective and the sample is controlled by an autofocussystem not shown herein but described in U.S. Pat. No. 5,978,074.

[0031] The returning reflected beams 34 and 36 are reflected by beamsplitter 52. A filter 54 is provided to remove the pump beam light 34allowing the probe beam light to fall on the photodetector 60. Detector60 provides an output signal which is proportional to the power of thereflected probe beam 36. Detector 60 is arranged to be underfilled sothat its output can be insensitive to any changes in beam diameter orposition. In the preferred embodiment, detector 60 is a quad cellgenerating four separate outputs. When used to measure reflected beampower, the output of all four quadrants are summed. As described in U.S.Pat. No. 5,978,074, the apparatus can also be operated to measure beamdeflection. In the latter case, the output of one adjacent pair ofquadrants is summed and subtracted from the sum of the remaining pair ofquadrants.

[0032] The output of the photodetector 60 is passed through a low passfilter 72 before reaching processor 42. One function of filter 72 is topass a signal to the processor 42 proportional to the DC power of thereflected probe. A portion of filter 72 also functions to isolate thechanges in power of the reflected probe beam which are synchronous withthe pump beam modulation frequency. In the preferred embodiment, thefilter 72 includes a lock-in detector for monitoring the magnitude andphase of the periodic reflectivity signal. Because the modulationfrequency of pump laser can be so high, it is preferable to provide aninitial down-mixing stage for reducing the frequency of detection.Further details of the preferred filter and alternatives are describedin U.S. Pat. No. 5,978,074. For example, it would be possible to use amodulated pump beam to obtain an optically heterodyned signal asdescribed in U.S. Pat. No. 5,206,710, incorporated herein by reference.

[0033] To insure proper repeatability of the measurements, the signalsmust be normalized in the processor. As noted above, the DC reflectivityof the probe beam is derived from detector 60. In addition, the DCoutput powers of the pump and probe lasers are monitored by incidentpower detectors (not shown) and provided to the processor. The signalsare further normalized by taking a measurement of the power of the pumpbeam 34 after it has been reflected by another detector (not shown).This measurement is used to determine the amount of pump energy whichhas been absorbed in the sample. The DC signal for both the incidentpump and probe beam powers as well as the reflected beam powers are usedto correct for laser intensity fluctuations and absorption andreflection variations in the samples. In addition, the signals can beused to help calculate sample parameters.

[0034] In accordance with the subject invention, in addition to thethermal wave measurement system, a separate spectroscopic measurementsystem is also included. This additional system includes a polychromaticor white light source 80. The white light source can be defined by asingle broadband lamp, such as a xenon arc lamp. Alternatively, thewhite light source could be defined by two or more lamps such as a xenonarc lamp to cover of the visible light ranges and a separate deuteriumlamp to cover the ultraviolet ranges.

[0035] The output from the white light source 80 is a polychromaticprobe the beam 82. The beam can be redirected by a splitter 84 towardsthe sample. The beam 82 is focused onto the sample by microscopeobjective 50. The reflected beam is redirected by splitter 86 to aspectrometer 88. The spectrometer can be of any type commonly known andused in the prior art. In the illustrated embodiment, the spectrometerincludes a curved grating 90 which functions to angularly spread thebeam as a function of wavelengths. A photodetector 92 is provided formeasuring the beam. Photodetector 92 is typically a photodiode arraywith different wavelengths or colors falling on each element in thearray. Other alternative detectors would include a CCD camera orphotomultiplier. It should be noted that it is within the scope of thisinvention to use a monochrometer and obtain measurements serially (onewavelength at a time) using a single detector element.

[0036] The output of detector 92 is supplied to the processor 42. Whenthe polychromatic light beam 82 follows the path discussed above, theoutput of detector 92 would correspond to the reflectance of the sample.In accordance with the subject invention, polychromatic light beam 82can also be used to obtain spectroscopic ellipsometric measurements.

[0037] In order to obtain spectroscopic ellipsometric measurements, abeam splitter 102 can be placed in the path of the polychromatic lightbeam 82. Beams splitter 102 redirects the beam through polarizer 106 tocreate a known polarization state. Polarizer 106 can be a linearpolarizer made from a quartz Rochon prism. The polarized probe beam isfocused onto the sample 20 by a curved mirror 108. The beam strikes thesample at an angle on the order of 70 degrees to the normal to maximizesensitivity. Based upon well-known ellipsometric principles, thereflected beam will generally have a mixed linear and circularpolarization state after interacting with the sample, as compared to thelinear polarization state of the incoming beam. The reflected beam isredirected by mirror 110 through a rotating compensator 112. Compensator112 introduces a relative phase delay or phase retardation between apair of mutually orthogonal polarized optical beam components. Theamount of phase retardation is a function of the wavelength, thedispersion characteristics of the material used to form the compensatorand the thickness of the compensator. The compensator is rotated bymotor 116 at an angular velocity ω about an axis substantially parallelto the propagation direction of the beam. In the preferred embodiments,compensator 112 is a bi-plate compensator constructed of two parallelplates of anisotropic (usually birefringent) material, such as quartzcrystals of opposite handedness, where the fast axes of the two platesare perpendicular to each other and the thicknesses are nearly equal,differing only by enough to realize a net first-order retardation overthe wavelength range of interest.

[0038] After passing through the compensator 112, the beam interactswith the analyzer 120. Analyzer 120 service to mix a polarization statesof the beam. In this embodiment, analyzer 120 is another linearpolarizer. The rotating compensator spectroscopic ellipsometerillustrated herein is described in greater detail in U.S. Pat. No.5,973,787 assigned to the same assignee and incorporated herein byreference. While a rotating compensator ellipsometer is disclosed, thescope of the subject invention is intended to include any of the otherconventional spectroscopic ellipsometer configurations. These wouldinclude rotating analyzer systems as well as fixed element systems thatrely on photoelastic modulators for retardation.

[0039] After the beam passes analyzer 120 it is reflected by beamsplitter 130 and directed to the spectrometer 88. As noted above,grating 90 disperses the beam onto the array detector 92. The measuredoutput from the spectrometer corresponds to the change in polarizationstate of the beam and from this information, the traditionalellipsometric parameters Ψ and Δ can be derived.

[0040] The optical layout in FIG. 1 is intended to illustrate how both athermal wave detection system and a spectroscopic detection system, andin particular, a spectroscopic ellipsometric system might be employed toobtain measurements at generally the same spot on the surface of thesample and in a near contemporaneous fashion. In this manner, thecombination of the measurements results will produce a more accurateresult.

[0041] The combination of the two metrology devices in a single tool inaddition to providing more accurate results provides economic benefitsas well. For example, a single tool has a smaller footprint andtherefore takes up less space in the semiconductor fab. By combiningtechnologies in a single tool, costs can be reduced by eliminatingduplicate subsystems such as wafer handlers and computers. Finally, thecombination can simplify and streamline decision making since theinformation from the two measurement modalities can be coordinatedinstead of producing conflicting results which can occur if two separatedevices were used.

[0042] It is believed that a combination thermal wave and spectroscopicinspection device will be particularly useful for analyzing ionimplantation in semiconductor samples. To determine the efficacy of thisapproach, experiments were run to obtain data on similarly preparedwafers on the assignee's Therma-Probe thermal wave device (ModelTP-500), as well as two of assignee's Opti-Probe models, the 3260 forthe broadband spectroscopic measurements and the 5240 to obtainbroadband spectroscopic ellipsometric measurements.

[0043] In these experiments, a total of 23 wafers each having a diameterof 150 mm were prepared on a GSD-600 ion implanter using P⁺ ions at 100keV energy or As⁺ ions at 30 keV. Each wafer had a uniform dose with 20wafers having the P⁺ dose within the range from 1e10 to 1e16 ions/cm²and three wafers having the As⁺ ion dose of 5.4e12±5% ions/cm². Thewafers were not treated for thermal annealing of the crystalline damage.21-point map broadband (210-780 nm) spectroscopic ellipsometry (SE)measurements were performed on the Opti-Probe 5240 and thermal wavemeasurements were done on the Therma-Probe 500 (TP-500) ion implantmonitoring tool. For added comparison, broad band (190-780 nm) spectralreflectance (SR) was done on an Opti-Probe 3260 (OP-3260).

[0044] As a part of this investigation, measurement recipes weredeveloped for the OP-5240 and OP-3260 while the ion implant monitoringrecipes were readily available on the TP-500. For ion implant monitoringpurposes, a simple model was applied with a single damage layer onundoped crystalline silicon and with an overlayer of thermal oxide. Thewafers under study had a nominal 100 Å of thermal oxide deposited priorto ion implantation. The optical properties of thermal oxide are wellknown and the standard library lookup table values were used for theoxide dispersion while the thickness of the top oxide layer was allowedto vary as a fitting parameter. A critical point model with fiveharmonic oscillators was used for the optical dispersion of the damagelayer. Initial recipe development involved fitting of the experimentaldata with variable dispersion for the damage layer as well as the damagelayer and the oxide thicknesses.

[0045] The phase transition from crystalline-like to amorphous-likesilicon was observed between doses 2e14 and 4e14 ions/cm² for thePhosphorous implant at 100 keV. The dose range can then be divided intoa low dose region (1e 10-1e14), a medium dose region (1e14-6e14) and ahigh dose region (6e14 -1e16). A separate recipe is needed for the curvefitting of each dose region due to the limited dynamic range of thefitting algorithm. FIG. 2 shows the results of curve fitting for thefull range of ion doses characterized with spectroscopic ellipsometry.FIG. 3 has the results for the thermal wave technique. Thecharacteristic low sensitivity plateau region around the 700 TW unit ismissing in the SE results. Furthermore, the negative slope at higherdoses, which is related to the optical interference effects in theamorphous layer, is not an issue for the SE measurements.

[0046] In addition to the dose sensitivity, the instrumental noise needsto be included in any analysis of implant monitoring capability. Thedose sensitivity S in units of %-change in a monitoring parameter per%-change in ion dose can be estimated from$S = \frac{\left( {P_{2} - P_{1}} \right)/\left( {P_{2} + P_{1}} \right)}{\left( {D_{2} - D_{1}} \right)/\left( {D_{2} + D_{1}} \right)}$

[0047] where P refers to the monitoring parameter, D is the ion dose,and the subscript 1 and 2 refer to the samples in question.

[0048] To estimate the noise for each technology, the short termrepeatability of map measurements was determined within a one to twohour period with 10 cycles of wafer loading, and the long termrepeatability was estimated from map measurements repeated once a dayfor five consecutive days. The repeatability is defined as the standarddeviation at 1-σ and the percent notation %σ=1−σ/mean mean is used here.For each technology, the detection limit DL in %-change in dose can thenbe estimated from ${DL} = \frac{\% \sigma}{S}$

[0049] The monitoring results for four sample wafers are illustrated inFIG. 4 with the statistical process control (SPC) plots. The data inTables 1 and 2 are analyzed to extract the low dose and high dosedetection limits in %-dose for each technology. The spectroscopicellipsometry technology shows superior performance in the high dosedetection with a factor of 30 improvement in comparison to the industrystandard thermal wave technology. For low dose detection the SEperformance is comparable to TP-500.

[0050] Table 1 below illustrates the long term detection at high dosewith each technology. The monitoring parameter for TP-500 is the thermalwave signal in TW units and for the OP-tools the monitoring parameter isthe damage layer thickness in Å. HIGH DOSE TP-500 OP-5240* OP-3260 P 100keV Average 7825.93 1120.36 1173.265 2.00E+15 1-σ 205.066 0.271 0.802 %1-σ 2.620 0.024 0.068 P 100 keV Average 24667.05 2793.66 2795.6494.00E+15 1-σ 62.734 0.568 0.589 % 1-σ 0.254 0.020 0.021 Sensitivity(%-per-%) 1.555 1.283 1.226 Detection limit (% 1.685 0.019 0.056 dose)

[0051] Table 2 below shows the long term detection at low dose with eachtechnology. The monitoring parameter for TP-500 is the thermal wavesignal in TW units and for the OP-tools the monitoring parameter is thedamage layer thickness in Å. LOW DOSE TP-500 OP-5240* OP-3260 As 30 keVAverage 700.61 44.45 42.825 5.13E+12 1-σ 0.265 0.130 0.261 % 1-σ 0.0380.293 0.610 As 30 keV Average 704.42 47.71 44.887 5.40E+12 1-σ 0.3820.235 0.175 % 1-σ 0.054 0.494 0.390 Sensitivity (%-per-%) 0.106 1.3790.917 Detection limit (% 0.512 0.358 0.666 dose)

[0052] As can be seen from the above data, both the thermal wavemeasurements and the spectroscopic measurements can provide usefulinformation about ion implantation in semiconductor wafers. Inaccordance with the subject invention, this data can be combined inorder to improve the analysis of the sample and reduced ambiguities.

[0053] There are a number of approaches which are available to combinedata from different technologies. One could use the data independentlyto arrive at separate approximations of the ion implantation dose of thesample and then derive a final result by taking a weighted average ofthe independent solutions. Preferably, a more robust analysis will beperformed that combines data from both measurements in iterative processto reach a best fit solution. Such iterative approaches for combiningdata from multiple measurements are now more well known in the metrologyfield. For example see “Simultaneous Measurement of Six Layers in aSilicon on Insulator Film Stack Using Spectrophotometry and Beam ProfileReflectometry,” Leng, et. al, Journal of Applied Physics, Vol 81, No. 8,Apr. 15, 1997.

[0054] The subject invention is not limited to the particular algorithmused to derive the characteristics of the individual layers. In additionto the more conventional least square fitting routines, alternativeapproaches can be used. For example, the high level of computing powernow available permits approaches to be utilized which include geneticalgorithms. One example of the use of genetic algorithms to determinethe thickness of thin film layers can be found in “Using GeneticAlgorithms with Local Search for Thin Film Metrology,” Land, et. al.,Proceeding of the Seventh International Conference on GeneticAlgorithms, July 19-23, page 537, 1997. See also, U.S. Pat. No.5,864,633, incorporated herein by reference.

[0055] In one particular implementation of the subject invention, thethermal wave signals and the spectroscopic ellipsometer signals arecombined to analyze the extent of damage induced by ion implantation. Asnoted above, the spectroscopic ellipsometer signals are particularlyuseful in characterizing the extent of the amorphous layer created bythe ion implantation process. Conversely, the thermal wave data isparticularly sensitive to the overall damage layer. By properlycombining the data with suitable algorithms, a full characterization ofthe ion implant damage as a function of depth can be achieved.Alternatively, the spectroscopic reflectivity measurements could becombined with the thermal wave measurements to perform this analysis.

[0056] In addition to analyzing ion implantation processes, thecombination of thermal wave and spectroscopic ellipsometric measurementscan be used to reduce ambiguities in measurements associated with otheraspects of semiconductor fabrication. For example, this combinationcould be used to analyze characteristics of various thin films of thesample. Such thin films can include oxides, nitrides and metals.Information obtained from the two or more independent measurementsallows one to more accurately determined the characteristics of thoselayers. Those characteristics could include, for example, thickness,index of refraction, extinction coefficient, density and thermalconductivity.

[0057] Is also within the scope of the subject invention to combineadditional technologies to the measurement beyond those illustrated inFIG. 1. For example, measurement technologies such as the assignee'sproprietary beam profile reflectometry and beam profile ellipsometrycould be used. The addition of such further measurement modules isdescribed in assignee's prior U.S. Pat. No. 5,978,074.

[0058] While the subject invention has been described with reference toa preferred embodiment, various changes and modifications could be madetherein, by one skilled in the art, without varying from the scope andspirit of the subject invention as defined by the appended claims.

We claim:
 1. A method of evaluating characteristics of a samplecomprising the steps of: periodically exciting a region on the surfaceof the sample; monitoring the modulated optical reflectivity induced bysaid periodic excitation and generating first output signals in responsethereto; obtaining spectroscopic ellipsometric information from the sameregion on the sample surface and generating second output signals inresponse thereto; and evaluating the characteristics of the sample basedon the first and second output signals.
 2. A method as recited in claim1 wherein said step of obtaining spectroscopic ellipsometric informationcomprises the further steps of: directing a polychromatic probe beamhaving a known polarization to reflect off the surface of the sample;and monitoring the change in polarization state of the probe beaminduced by reflection at a plurality of wavelengths.
 3. A method asrecited in claim 2 wherein said sample is a semiconductor wafer whereinthe upper region thereof has been implanted with a high dose of ionssufficient to create a damaged region within which lies an amorphouslayer and wherein said first output signals are used to determine theextent of the damaged region and the second output signals are used todetermine the extent of the amorphous layer.
 4. A method of evaluatingcharacteristics of a semiconductor wafer wherein the upper regionthereof has been implanted with a high dose of ions sufficient to createa damaged region within which lies an amorphous layer, said methodcomprising the steps of: periodically exciting a region on the surfaceof the wafer; monitoring the modulated optical reflectivity induced bysaid periodic excitation and generating first output signals in responsethereto; obtaining spectroscopic optical information from the sameregion on the sample surface and generating second output signals inresponse thereto; and determining the extent of the damaged region usingthe first output signals and determining the extent of the amorphouslayer with the second output signals.
 5. A method as recited in claim 4wherein said step of obtaining spectroscopic optical informationcomprises the further steps of: directing a polychromatic probe beamhaving a known polarization to reflect off the surface of the sample;and monitoring the change in polarization state of the probe beaminduced by reflection at a plurality of wavelengths.
 6. A method asrecited in claim 4 wherein said step of obtaining spectroscopic opticalinformation includes the steps of: directing a polychromatic light beamonto the surface of the sample; and measuring the intensity of thereflected polychromatic light beam at a plurality of wavelengths.
 7. Anapparatus for evaluating characteristics of a sample: means forperiodically exciting a region on the surface of the sample; means formonitoring the modulated optical reflectivity induced by said periodicexcitation and generating first output signals in response thereto;means for obtaining spectroscopic ellipsometric information from thesame region on the sample surface and generating second output signalsin response thereto; and a processor for evaluating the characteristicsof the sample based on the first and second output signals.
 8. Anapparatus as recited in claim 7 wherein said means for obtainingspectroscopic ellipsometric information comprises: a polychromatic probebeam having a known polarization which is directed to reflect off thesample; and means for monitoring the change in polarization state of theprobe beam induced by the reflection off the sample at a plurality ofwavelengths.
 9. A method of evaluating characteristics of a samplecomprising the steps of: periodically exciting a region on the surfaceof the sample; monitoring the modulated optical reflectivity induced bysaid periodic excitation and generating first output signals in responsethereto; directing a polychromatic light beam having a knownpolarization state within the same region on the surface of the sample;measuring the change in polarization state of the reflectedpolychromatic light beam and generating second output signals inresponse thereto; and evaluating the characteristics of the sample basedon the first and second output signals.
 10. An apparatus for evaluatingthe characteristics of a sample, comprising: an intensity modulated pumplaser beam, said pump laser beam being directed to a spot on the surfaceof the sample for periodically exciting the sample; a probe laser beambeing directed to a spot on the surface of the sample within a regionwhich has been periodically excited and is reflected therefrom; adetector for measuring the power of the reflected probe laser beam andgenerating a first output signal in response thereto; a broadbandpolychromatic light source for generating a polychromatic probe beamhaving a known polarization state, said polychromatic probe beam beingdirected to reflect off a spot on the surface of the sample; an analyzerfor monitoring the change in polarization state of the reflectedpolychromatic probe beam and generating a plurality of second outputsignals corresponding to a plurality of different wavelengths within thepolychromatic probe beam; and a processor for filtering the first outputsignal to provide a measure of the magnitude or phase of the modulatedoptical reflectivity of the sample, said processor further functioningto monitor the second output signals with the first and second outputsignals being used to evaluate the characteristics of the sample.
 11. Anapparatus as recited in claim 10 wherein the probe beam from thepolychromatic light source is directed to the same location as the probelaser beam is directed.
 12. An apparatus as recited in claim 10 furtherincluding a steering means for adjusting the lateral separation betweenthe pump and probe laser beam spots on the surface of the sample andwherein a plurality of measurements are taken at different separationsbetween the pump and probe laser beam spots.
 13. An apparatus as recitedin claim 10 further including a means for varying the modulationfrequency of the pump laser beam and wherein a plurality of measurementsare taken at different modulation frequencies.
 14. A method forevaluating the characteristics of a sample comprising the steps of:directing an intensity modulated pump laser beam to a spot on thesurface of the sample for periodically exciting the sample; directing aprobe laser beam to a spot on the surface of the sample within a regionthat has been periodically excited and is reflected therefrom; measuringthe power of the reflected probe beam and generating an output signal inresponse thereto; filtering the output signals to provide a measure ofthe magnitude or phase of the modulated optical reflectivity of thesample; directing a broadband, polychromatic light beam having a knownpolarization state onto a spot on the surface of the sample; monitoringthe change in polarization state of the reflected polychromatic lightbeam and generating a plurality of second output signals correspondingto a plurality of different wavelengths within the polychromatic beam;and evaluating the characteristics of the sample using the modulatedoptical reflectivity measurements and the polarization state changes atdifferent wavelengths.
 15. A method as recited in claim 14 furtherincluding the step of varying the modulation frequency of the pump laserand measuring the power of the reflected probe laser beam at a pluralityof modulation frequencies and using the measurements to characterize thesample.
 16. A method as recited in claim 14 further including the stepof varying the separation between pump and probe laser beam spots on thesample surface and measuring the power of the reflected probe laser beamat a plurality of separations and using the measurements to characterizethe sample.
 17. A method as recited in claim 14 wherein said sample is asemiconductor wafer wherein the upper region thereof has been implantedwith a high dose of ions sufficient to create a damaged region withinwhich lies an amorphous layer and wherein said first output signals areused to determine the extent of the damaged region and the second outputsignals are used to determine the extent of the amorphous layer.